Widely used types of ink for inkjet recording apparatuses are a dye ink, which contains dye as a color material, and a pigment ink, which contains pigment as a color material. Forming an image on a recording medium using a pigment ink leads to a phenomenon of coloring of specular reflection light, which is light reflected by the formed image. For example, when an image formed by this kind of recording apparatus is placed under a light source such as a spotlight, although the spotlight emits achromatic light, this light turns into colored specular reflection light after being reflected on the recording medium.
Especially, in a color image, specular reflection light tends to be colored magenta on a region with a cyan ink laid on a large part of the region, while specular reflection light tends to be colored yellow on a monochrome image as a whole. Further, such coloring of specular reflection light tends to change in an iridescent manner according to a change in an ink amount depending on regions in an image. Occurrence of this coloring of specular reflection light results in deterioration of the image quality due to a difference between the color of the specular reflection light and the color of the diffused light.
Now, a method of evaluating coloring of specular reflection light (Japanese Patent Application Laid-Open No. 2006-177797) will be described with reference to FIG. 1. A measurement sample 101 is irradiated with light by a light source 102 from a predetermined angle, and the specular reflection light reflected by the measurement sample 101 is detected by a light receiver 103. The light receiver 103 detects tristimulus values XxYxZx in the International Commission on Illumination (CIE) XYZ color system. The a*b* in the CIE L*a*b color system is calculated based on a difference between the detected XxYxZx and the tristimulus values XxYxZx of a sample that does not cause bronzing (for example, a black polished glass plate on which the wavelength dispersion of the reflective index is small). Color saturation C*, which is expressed by this a*b*, indicates the degree how much the specular reflection light is colored. Less colored specular reflection light outputs low C*, and, for example, the value of C* becomes zero for a sample that does not cause coloring of specular reflection light (in other words, C* is positioned on the origin point on the a*b* plane). Bronzing and thin-film interference are known as reasons that specular reflection light is colored as mentioned above.
Bronzing is a phenomenon that occurs due to wavelength dependency of reflection on an interface of a formed image. It is known that each ink has a unique color to which the color of the ink is changed by a bronzing phenomenon. For example, specular reflection light is colored magenta on an image region formed with a cyan ink. A main cause of a bronzing phenomenon is wavelength dependency of reflection on an interface between an air layer and an ink layer. Therefore, there is known a method reducing bronzing by further discharging a yellow ink after forming an image to cover the image region formed with a cyan ink or a magenta ink, which have relatively high wavelength dependency, with a yellow ink, which has comparatively low wavelength dependency (Japanese Patent Application Laid-Open No. 2004-181688). This patent literature further discusses that the discharge amount of a yellow ink is relatively reduced in the range of hue angles 180 degrees to 360 degrees (the hue region from cyan to magenta) in the L*a*b space.
However, the method of Japanese Patent Application Laid-Open No. 2004-181688 results in a reduction in the color gamut, if a yellow ink is applied to cover and overcoat the region from cyan to blue to magenta in the L*a*b space where a cyan ink and a magenta ink are largely used. Especially, yellow is in a complementary color relationship with blue, and therefore blue is subject to a striking reduction in the color gamut thereof. On the other hand, reducing the discharge amount of a yellow ink is effective to maintain the color gamut, but instead induces a noticeable bronzing phenomenon.
Another possible measure against coloring of specular reflection light is a method of using an achromatic color material, which is an ink containing no color material, as a recording agent with which an image is overcoated. The degree of bronzing is indicated by tristimulus values, and an achromatic color material has extremely small tristimulus values. In addition, a transparent achromatic color material does not affect color development. Therefore, use of an achromatic color material is expected to more effectively reduce coloring of specular reflection light without sacrificing the color gamut.
However, overcoating an image with use of an achromatic color material results in a change in coloring of specular reflection light according to the ink amount of the achromatic color material (achromatic color material amount), since a thin-film interference occurs due to an optical path difference of reflected light between the upper layer and the lower layer of the achromatic color material layer. FIG. 2 is a graph constructed by overcoating a solid surface of a cyan ink with an achromatic color material while changing the color material amount of the achromatic color material, evaluating the coloring of specular reflection light at this time with use of the method discussed in Japanese Patent Application Laid-Open No. 2006-177797, and then plotting the evaluation results on the a*b* plane. The numerical values on the graph of FIG. 2 indicate the achromatic color material amount. This graph shows that the coloring of specular reflection light reflected by the solid surface of the cyan ink is located in the magenta hue under the influence of bronzing, and the coloring is rotated in the clockwise direction on the a*b* plane according to an increase in the achromatic color material amount. In other words, overcoating a chromatic color material with an achromatic color material does not necessarily reduce coloring of specular reflection light, and coloring varies depending on the achromatic color material amount.
Further, an experiment has revealed that coloring also varies depending on the type of a chromatic color material laid under an achromatic color material. For example, coloring caused when a predetermined amount of a clear ink is overlaid on a solid surface of a cyan ink is different from coloring caused when the same amount of the clear ink is overlaid on a solid surface of a magenta ink. This means that just uniformly overcoating a color ink with a predetermined amount of an achromatic color material cannot sufficiently reduce coloring of specular reflection light.